Gyratory ball mill



May 2, 1961 A. w. FAHRENWALD 2,982,485

GYRATORY BALL MILL Filed Sept. 8, 1958 4 Sheets-Sheet 1 P ig. 1.

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INVENTOR. l ARTHUR FAHRENWALD BY f A. w. FAHRENWALD 2,982,485

GYRATORY BALL MILL May 2, 1961 Filed Sept. 8, 1958 4 Sheets-Sheet 2 INVENTOR.

ARTHUR FAHRENWALD www,

May 2, 1961 A. W. FAHRENWALD 2,982,485

GYRATORY BALL MILL Filed Sept. 8, 1958 4 Sheets-Sheet 5 Fig. 4.

GM-eoo MESH QUARTZ PER WATT-MIN. v

XX GM 20o MESH QUARTZ PER MIN.

XXX POWER IMPUWATTS XXX XXX

I l l l O 280 3230 ZIO 340 580 GYRATION PER MIN "N" ARTHUR W FAHRE NWALD BY mw @it q en-,200 QUARTZ MiNr-m0 200 M-POWER :NPUT

INVENTOR.

f May 2, 1961 A. w. FAHRENWALD GYRATORY BALL MILL Filed Sept.. 8, 1958 4 Sheets-Sheet 4 INVENTOR.

United States Patent C) Y 2,982,4ss

GYRATORY BALL MILL Anhui w. Fahronwold, 117 N. Howard, Moscow, Idaho 'Filed Sept. 8, 1958, Ser. No. 759,672

s Claims. (ci. z41-17s) My invention relates to` improvements in a gyratory ball mill for grinding purposes. By gyratory I mean a mill wherein the movement of the shell or container for the balls is a gyration in a horizontal plane on a radius that is small compared to the container diameter.

It is the purpose of my invention to provide a gyratory ball mill having a ball load confined in a gyrating shell wherein the physical characteristics, such as shell depth,-

ball load depth, shell diameter, direction of feed and radius of gyration are so related as to take advantage of optimum crushing relationships for e'icient reduction of the particle size. Y

A specific object of my invention is to provide a novel feed and discharge means for a gyratory ball mill operable to provide the minimum travel for the feed material with the material contained in each particle originally fed into themill being subjected to an increasing number of crushing blows. as the material is broken up into an increasing number of particles. 4

Other and more detailed objects and advantages will appear from the following description and the accompanying drawings wherein a preferred form of my invention is illustrated. The drawings and description are illustrative, however, and are not intended to limit my invention except insofar as it is limited by the claims.

In the drawings:

Figure l is a side View of the ball mill with parts broken away to show the supporting structure in sec-y Figure 7 is a plan sectional view taken on the linev 7-7 of Figure 6;

rAs illustrated by Figures 1-3, the gyratory ball mill comprises a cylindrical shell 10 that is supported upon a base 11.

The shell carries a load of balls 9. The base 11 is supported by a stem 12 which is eccentrically mounted by bearings 13 on a drive member 14. A bearing shell 15 is bolted to the member 14 to position the stem 12 eccentrically with respect to the axis of rotation of the member 14. The member 14 is mounted on a shaft 16 which is supported by bearings 17 in a bearing sleeve 18. The bearing sleeve 18 is mounted by a cross piece 19 of the main frame 20. The drive member 14, shown as a pulley, has a weight 21 thereon diametrically opposite the bearing shell 15.

The member 14 is connected by a belt 22 to a pulley 23 on the shaft 24 of a motor 25. The motor 25 is mounted on the frame by an L-bracket 26 slidable Patented May 2, 1961 on the mounting studs 27 and adjustable to keepthe belt 22 tight by a screw 28 in a block 29 on thebracket 26. f

The base 11 and shell 10 are restrained against'rotation about the axis of the shell 10 bydiametrically opposed springs 30 and 31 that 'are secured at one end to arms 32 and 33 on the base 11 and have their opposite ends hooked to brackets 34' and 35 on the frame 20. More springs may be used if necessary, since the motion .of the shell 10 desired is a non-rotating, gyratory movement wherein every point of the shell makes a circular gyration in a horizontal plane.

The shell 10 is closed at the 'top by a cover memberl 36 that also clamps an annular discharge trough 37 down' on the'top edge of the shell 10. The cover member is;

held' down by four bolts 39. The trough 37 has an inner lip 38 spaced below the cover member 36 and riding on the top edge of the shell 10. The cover 36 has a central inlet ring 41 for supplying material to be broken up. The trough 37 has an outlet 42. Note that the' trough 37 increases in depth from a point opposite the outlet 42 to the outlet so that finished material passing over the lip 3S vbetween the cover 36 and the lip 38 will flow to the outlet 42.

The cover 36 has a replaceable grate'43 that forms its` bottom wall and contines the balls, when in motion,` This grate 43 has openings' 44 therein large enough to pass the particles being broken' against moving up too far.

up, but openings 44 are considerably smaller than the balls 9. through the grate 43. They may thereafter be thrown'up through the grate openings 44 but will work outwardly' and, again and again, enter the grinding area until iinally reaching the lip 38. The grate 43 may be secured to the cover member 36 in any suitable manner, as' by screws 45. Since the grate is worn by the impact of balls against it, replacement must takeplace.

ber 36. It will be noted that the shell 10 has a bevelled surface 46 between the cylindrical wall and the bottom` thereof. It is possible, without this surface 46, for a bottom layerof balls to wedge in place and thus put that much ofthe available grinding area within the shell 10 outof use. The bevelled surface 46 successfully obviates this diliculty.V Y

A gyratory mill constructed as just described, mustV include a Ycharge of balls particularly related in the V01-, ume the charge occupies to the volume of the space.in

the shell 10 below the grate 43. I have found that maximum grinding occurs `only when the charge of balls isl such that when gyrating on a certain radius, the shell is lled with the moving balls and they are striking against If the height of ball load in the shell 10 is such that substanthe grate 43 substantiallyvthroughout its area.

tially the entire space is lled when the'balls are vstationary, then when the 'shell 10 is gyrated the entire loadv moves substantially as a single mass with'the shell and.

the optimum obtained when the ball load is of the proper.

height in the shell 10.

I have discovered thatA there is an optimum range ofv volume of ball load measured in 'percentA of volume of the space within the shell available for balls. With shell volume h equal to the optimum volume of ball load is between 60% and 80%, the peak being reached at about 70%.

The particles.must enter the grinding area- The life of a grate is only a fraction of the life of the rest of .the cover mem.

In speaking of volume of ball load, I f' mean total of the spaces Aoccupied by the balls, plus the spaces between balls, plus the spaces between balls and the surrounding walls of the shell up to the top level of the balls when they are' spread out in Vthe container substantially level. Figure 4 of the drawings `shows a curve #l illustrating the eiciency (grams of 200 mesh quartz produced per watt minute); a curve #2 illustrating the capacity (grams of 200 mesh quartz produced per minute); and a curve #3 illustrating the required power input-watts to operate the mill, for ball loads of 40% to 80% of shell volume. These tests were carried out in a laboratory size mill having a diameter of 12 inches, a shell height of 3% inches, a radius of gyration of l inch. The gyration rate was 340 gyrations per minute. The ball sizes ranged from inch in diameter to 3%: inch in diameter. The material fed into the mill was quartz in particle sizes that would pass through a 14 mesh screen and be retained on a 48 mesh screen. The material was ground wet 70% solids. It is to be noted that both capacity and efficiency rose slowly from 40% ball load to 50% ball load, then quite rapidly from 50% to 60% ball load, then more slowly from 60% ball load to about 70%, and the drop rate increased from about 70% ball load until at 80% the drop rate was quite rapid.

Observation of the operation of the mill through a transparent cover brought out the fact that in operation the entire space within the shell seemed filled with balls in violent agitation. A substantial pressure upward against the cover was evident, showing that the impacts of the balls were upward as well as in other directions, indicating the advantage of the confined space when the ball load was of the proper percentage of shell volume.

An important characteristic in the mill operation is the feed from center to periphery in an upright cylinder. The balls used. particularly for dry grinding, should be of higher specific gravity than the material being ground so that the particles, when ground, work up to the top and pass over the lip of the discharge trough. By causing the particles entering the mill to move radially of the mill the distance travelled by a unit of feed material is at a minimum. The rate of travel of that unit of material toward the exit rim decreases as it approaches the rim. The number of particles in a particular unit of feed material starts to increase the moment it enters the field of moving balls. Likewise with this center to rim feed, the available ball impacts per unit of feed increases from center to rim of the mill. There are many more balls in a circle of the distance out from the center toward the rim than there are in a circle 1A; of the distance out from the center toward the rim. The radial direction of feed is therefore an important advantage in this gyratory ball mill.

I have also discovered that in order to produce the ball action which must be present in the mill to produce the high efficiency and capacity shown, it is necessary tov ball load (within the optimum percentage hereinbefore' outlined) is attained only when the product obtained through multiplying the number of gyrations per minute by the radius of gyration expressed in inches exceeds 280. In the graph Figure 5, this is clearly illustrated. Power input, which reflects grinding capacity, is indicated on the vertical line and the number of gyrations per minute on the horizontal line. The radius of gyration used was one inch.. There is a definite bend upward in the curve (plotted from test results taken with a radius of gyration of one inch) when the gyrations go above 280 per minute. The curve continues almost as a straight line above that number of g.p.m. As a practical matter, when the mill is gyrated at such a rate that the product of the g.p.m. and radius of gyration exceeds 280 the power draft of the mill is a lineal functionof the number of gyrations per minute.

4 Thus one can control the capacity of the mill closely by controlling the speed of gyration.

Tests have also been conducted where the number of gyrations per minute was kept constant and the radius of gyration was varied over a wide range. In these tests it was found that the grinding capacity was substantially a lineal function of the change in radius of gyration. That is, the curve plotted for several different radii of gyration, showing the output per minute, was substantially a straight line. A

The magnitude of ball impact varies directly with variation in the radius of gyration. Therefore it is obvious that in designing and operating a gyratory ball mill the radius of gyration will be selected on the basis of the size of particles to be fed into it. For coarse feed, a larger radius of gyration would be selected than for a smaller particle size. Then by selecting the proper number of gyrations per minute so that the product of g.p.m. times radius of gyration exceeds 280, the operator can obtain the desired optimum grinding conditions. Preferably I utilize a speed of gyration such that g,p.m. times radius of gyration is roughly 350.

Just as in the case of the conventional tumbling ball mill, such factors as ball size, hardness and specific gravity of the feed material, whether grinding is wet or dry, and specific gravity of the ball used, must be taken into account in designing and operating a gyratory ball mill. In all instances, however, the two fundamental relations hereinbefore discussed (ratio of ball load volume to mill volume and relation of rate of gyration and radius of gyration) must be followed if the exceptional advantages attained by my invention be realized.

I find that there is a fundamental relationship also between the diameter of the shell 10, ball load volume to shell volume ratio and the radius of gyration by which the optimum radius of gyration r may be selected for any diameter d of shell and assumed ball load lf/lz where 11' is the height of the static ball load and h" is the height of the space within the shell 10. This relationship is expressed by the formula @lati-2297 For example if d is assumed at 24 inches and r at 2 inches, then and under such circumstances a ball load of .70 of the shell volume is obtained. If, however, the radius of gyration were 6 inches, then and the ball load would be only 25% of the total shell volume. This is much too small to enable the ball load to swell under gyration to fill the available space in the l shell.

' In larger diameter mills of the order of six feet, the optimum radius of gyration for the correct ball load becomes about six inches. For most grinding the ball impacts with such a radius of gyration are of much greater force than is required to break up the particles. I have found that by using a central core 10a (see Figures 6 and 7) and perforating it as shown at 47 to allow the feed material to flow out through the core, I can then use a smaller radius of gyration and effectively obtain the desired ball action when both the core 10a and the space between the core 10a and the shell 10 are about 70% filled by the static ball load. With this arrangement to calculate the desired radius of gyration by the formula, the diameter can now be treated as either the core diameter or the distance from the core to the shell wall, whichever is the greater. If this core 10a is selected as two feet in a six foot shell 10, then the optimum radius of gyration is slightly less than 2 inches and the impact blows are ample to do the grinding. The core wall 10a is joined to the bottom of the shell by bevelled surfaces to avoid wedging of the bottom layer of balls. f I

In the grate and in the core wall 10a the apertures provided are ample for passage of the particles being ground, but substantially smaller than the diameter of the balls employed. The confinement of the balls within a space that is small enough to be essentially lled with the balls Y when they are agitated by gyration at a speed high enough to make the product of the gyrations per minute times the radius of gyrations expressed in inches above 280 and preferably about 350, provides the most eiective utilization of the impacts of the balls to reduce the particle size of the material to be crushed. I nd that some additional advantage may be attained by graduating the ball size, using the smallest balls in the bottom layer and gradually increasing the ball size to the top layer. The balls when so arranged, retain their relative positions while in operation.

It is believed that the nature and advantages of my in-4 vention will be apparent from, the foregoing description.

Having described my invention. I claim:

l. A gyratory ball mill comprising a cylindrical shell, a cover therefor, a base for said shell supporting it wth the shell axis being in a vertical position, means operable to gyrate said support about a vertical axis, yielding means preventing rotation of said shell while it is gyrating, a charge of balls in said shell wherein the static volume of the charge is equal to 60% to 80% of the enclosed shell volume, the number of gyrations per minute multiplied by the gyratory radius expressed in inches being in the range of 280-350, the radius of gyration being determined by the formula:

d 2 d-2r 2 (2) h 2 )h wherein d is the shell diameter, r is the radius of gyration, h is the height of the static charge of balls and h is the height of the cylindrical shell, a feed inlet at the top central portion of said shell, an outlet passage for the reduced material between said shell and its cover extending circumferentially about said shell, and a collecting means on the outside of said shell receiving the reduced material from said outlet passage, wherein the cover has a grate therein provided with apertures too small to pass one of the balls but large enough to pass the particles being fed into the mill.

2. A gyratory ball mill comprising a cylindrical shell, a cover therefor, a base for said shell supporting it with the shell axis being in a vertical position, means operable to gyrate said support about a vertical axis,

yielding means preventing rotation of said shell while it is gyratng, a charge of balls in said shell wherein the static volume of the charge is equal to 60% to 80% 0f the enclosed shell volume, the number of gyrations per minute multiplied by the gyratory radius expressed in inches being in the range of 280-350, the radius of gyration being determined by the formula:

wherein d is the shell diameter, r is the radius of gyration, h is the height of the static charge of balls and h is the height of the cylindrical shell, a feed inlet at the top central portion of said shell, an outlet passage for 5 the reduced material between said shell and its cover extending circumferentially about said shell, and a collecting means on the outside of said shell receiving the reduced material from said outlet passage, wherein the shell has a core wall therein extending upwardly from the 10 bottom wall thereof and concentric with the shell, said core wall being provided with apertures to pass the material being reduced.l

3. A gyratory ball mill comprising a cylindrical shell, a cover therefor, a base for said shell supporting it with the shell axis being in a vertical pos'tion, means operable to gyrate said support about a vertical axis, yielding means preventing rotation of said shell while it is gyrating, a charge of balls in said shell wherein the static volume of the charge is equal to 60% to 80% of the enclosed shell volume, the number of gyrations per minute multiplied by the gyratory radius expressed in inches being in the range of 280-350, the radius of gyration being determined by the formula:

d 2 d-Zr Z (a) MM-2 )h wherein d is the shell diameter, r is the radius of gyration, iz is the height of the static charge of balls and h is the height of the cylindrical shell, a feed inlet at the top central portion of said shell, an outlet passage for the reduced material between said shell and its cover extending circumferentially about said shell, a collecting means on the outside of said shell receiving the reduced material from said outlet passage, and a grate interposed between the ball load and said cover, wherein the space below said grate within the shell constitutes the enclosed volume of the shell.

References Cited in the le of this patent UNITED STATES PATENTS 584,086 Woods June 8, 1897 849,545 Hunt Apr. 9, 1907 859,118 Schietler July 2, 1907 2,338,398 Bibolfni Jan. 4, 1944 2,540,358 Symons Feb. 6,1951

FOREIGN PATENTS 260,777 Germany June 6, 1913 667,333 Germany Nov. 9, 1938 707,525 Germany June 25, 1941 OTHER REFERENCES Fahrenwald: New Type of Grinding Mill, pages 93 to 97 of Rock Products, volume 54, Number 2, February 1951 (Chicago, Illinois, Maclean-Hunter Publishing Corporation), TN950.A3. 

